Potential evaporation dynamics over saturated bare soil and an open waters surface

Actual evaporation (Ea) can be calculated as a fraction of potential evaporation (PE), which refers to the evaporation
rate if supply water is unlimited. Potential evaporation depends on the available energy and the underlying
material, and different approaches to estimate potential evaporation exist nowadays. This study provides
a detailed analysis of the evaporation dynamics over fully saturated, sandy soil (PEs) and an open water
surface (PEw). Moreover, the performance of commonly used methods to estimate PE is assessed. At the basis of
these analyses is a lysimeter experiment in the Guanzhong Basin, China, which allowed a precise measurement
of PE with a very high temporal resolution. Temperature profiles in lysimeters and meteorological data were also
measured during the experiment. A comparison of PEs and PEw was carried out for seven consecutive days
(August 11th to 17th, 2016). Results show that PEw is smaller than PEs on a daily scale, with PEw rates being
bigger than PEs at night but smaller during daytime. Furthermore, the temporal dynamics of PEw lags 4–5 h
behind PEs. In accordance with the energy balance equation, PE dynamics are mainly governed by “available
energy”. The PE rates calculated by Penman-Monteith (PM) and Priestly-Taylor (PT) based on these measurements
were also evaluated. The measured PE is relatively well reproduced by PM and PT equations. Finally, the
effect of using different approaches to estimate PE on calculating Ea was tested by an integrated hydrological
model that calculates water flow in the unsaturated zone by solving the Richards equation. The relative differences
were up to 17.5%.